MIMO Systems
MIMO (multiple input multiple output) is an advanced antenna technique to improve spectral efficiency in mobile communication networks and thereby boost the overall capacity of the system. The MIMO technique uses a commonly known notation (M×N) to represent MIMO configuration in terms number of transmit (M) and receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2×1), (1×), (2×2), (4×2), (8×2) and (8×4). The configurations represented by (2×1) and (1×2) are special cases of MIMO and they correspond to transmit diversity and receiver diversity respectively. The configuration (2×2) is used in WCDMA release 7 and configurations ((4×4), (4×2)) are being defined in release 11.
Currently a four transmitting antenna (4Tx) transmission scheme for High Speed Downlink Packet Access (HSDPA) is discussed within 3GPP standardization (for example as described in “New WI: Four Branch MIMO transmission for HSDPA” by Ericsson, RP-111393; “4-Branch MIMO for HSDPA” by Ericsson, R1-111763; and “Common Pilot Design for Four Branch MIMO System” by Ericsson, R1-120352). Previous versions of the specification supported up to 2 TX antenna transmissions.
In order to support 4Tx MIMO transmissions, it is necessary to obtain four channel estimates in order to characterize each of the spatial layers, which will require new pilot signals. Pilots are needed for two main functionalities; channel state information (CSI) estimation through channel sounding, where rank, channel quality information (CQI) and precoding control index (PCI) are estimated, and channel estimation for demodulation purposes. The term pilot is also interchangeably called a reference signal in this application, but they are considered to have the same meaning. A pilot or reference signal is a known sequence of signals which are pre-defined or known to the UE in advance. Other commonly known terms which have the same meaning as ‘pilots’ are UE specific reference signal, dedicated reference signal, demodulation reference signal (DMRS), CSI reference signal (CSI-RS), cell specific reference signal (CRS) etc. The terms dedicated pilot, dedicated reference signal, demodulation reference signal or UE specific reference signal is used for a specific purpose in a particular direction to assist, for example, the demodulation of transmission using beamforming or for a specific UE or for a group of UEs or any combination thereof. On the other hand, a common pilot, common reference signal or cell specific reference signal is sent over the entire cell and is used for multiple purposes and is for use by all UEs e.g. measurements, demodulation etc.
Two different approaches are possible for 4-branch MIMO. The first is to use common pilots for both CSI and channel estimation for data demodulation. The second is to use common pilots for CSI estimation and additional high power pilots without precoding for channel estimation for data demodulation. In this context, “common pilots” refer to pilots that are made available to all users and which are transmitted without user specific beamforming.
Common pilots may be transmitted at instances in which legacy users (i.e. users that are not designed according to the latest releases of the specifications, such as users designed according to Release 7 MIMO and Release 99), who are not able to demodulate 4TX transmissions, are scheduled. These legacy users cannot make use of the energy in the common pilots. However, the energy in the additional common pilots will reduce the amount of energy available for High Speed-Physical Data Shared Channel (HS-PDSCH) scheduling to the legacy users. Moreover, the additional common pilots cause interference to these users. Therefore, to minimize performance impacts to non 4TX users, it is important that the power of the common pilots is reduced to a low value.
Unfortunately, however, with reduced pilot power of common pilots, the demodulation performance will be impacted. Hence, in addition to transmitting two common pilots with low power, two additional pilots with higher power are introduced for demodulation in a four branch MIMO system. These additional pilots can be referred to as “scheduled” or “demodulation” pilots. A base station (Node B) starts transmitting these additional pilots based on channel conditions and available power.
Use of Common Pilots for CSI Estimation and Data Demodulation
FIG. 1 illustrates a system 2 that uses common pilots for CSI estimation and data demodulation. On the Node B 4 (transmitting) side, known pilot symbols 6 are transmitted by transmitter module 8 for channel sounding. A receiver 10 in the UE 12 estimates channel quality (typically signal to interference plus noise ratio, SINR) from channel sounding using channel estimator 14, and computes a preferred precoding matrix W using precoder matrix calculator 16 and CQI for the next downlink transmission from the Node B 4. This information is conveyed by the UE 12 to the Node B 4 through a feedback channel 18 (e.g. a High Speed-Dedicated Physical Control Channel, HS-DPCCH). The Node B 4 processes this information and decides the precoding matrix and modulation (and some other parameters such as transport block size etc.) and conveys this information to the UE 12 through a downlink control channel. Data 20 is transmitted by the Node B 4 to the UE 12 with the modulation and coding rate indicated in the downlink control channel. The data 20 is pre-multiplied by precoding vector/matrix W in precoder block 22 before passing to the antenna ports 8. For data demodulation in the UE 12, the channel estimator 14 in the UE 12 estimates the channel H from the common pilot symbols, and a data detector 24 uses the channel estimate H to demodulate the data.
As noted above, common pilot-only solutions will have a negative impact on legacy users unless the power on the additional (common) pilots is minimal. FIG. 2 shows the performance of a pilot reduction scheme on the sector throughput with different numbers of users per sector. For this simulation, it is assumed that all the users are Release-7 MIMO capable with 2 receive antennas. The additional interference due to the additional third and fourth pilots is considered with different power levels. The pilot powers for the pilots transmitted by the first and second antennas are set to −10 dB and −13 dB respectively. It can be seen from FIG. 2 that as the power of the additional pilots is decreased, the impact on the system throughput performance is less. For example, if the relative pilot power (i.e. compared to the maximum downlink power in log scale) is around −19 dB then the impact on the legacy users is almost negligible.
However, if the power is minimal, then the demodulation performance of 4TX users will be adversely impacted. FIGS. 3 and 4 shows the Link level throughput for a UE with 3 different carrier-to-interference (C/I) ratios for 4×4 MIMO (FIG. 3) and 4×2 MIMO (FIG. 4). As discussed above, when operating with common pilots it will be necessary to minimize the power transmitted on the 3rd and 4th pilots in order to minimize impact to legacy users.
The figures show the performance with reduced pilot powers for the 3rd and 4th antennas with the pilot powers for the 1st and 2nd antennas being kept as −10 and −13 dB respectively. It can be observed that as the pilot powers are reduced, the performance degrades due to bad channel estimation for CQI and data demodulation. The degradation is severe at high C/I compared to low C/I. This is because at high C/I, there is a high probability of rank 3 and rank 4 transmissions and/or high data rates, which require a larger amount of pilot power energy. On the other hand, low data rates and/or rank selections, which occur at low C/I, can be demodulated with a lower amount of pilot energy (i.e. a higher traffic to pilot ratio).
Introduction of additional pilots when any 4 branch MIMO user is scheduled may cost some additional overhead and may not give benefit for all the scenarios. In reality, a high amount of pilot power is required when the UE is attempting to demodulate high data rates with high rank.
Use of Common Pilots for CSI Estimation and Additional Pilots for Data Demodulation
FIG. 5 shows a system 32 that uses common pilots for CSI estimation and additional pilots for data demodulation. The system 32 is similar to that shown in FIG. 1, with like reference numerals indicating similar features. Similar to the common pilot scheme, known pilot symbols 6 are used for channel sounding, the UE 12 conveys the preferred precoding matrix W, CQI through the feedback channel 18. For downlink data transmission, the Node B 4 uses this information and chooses the precoding matrix W, CQI and the transport block size. For data transmission, data 20 is multiplied by the precoding matrix W selected by the Node B 4 and transmitted. In addition to the data 20, additional pilots 34, that are similar to common pilots 6 but that are not precoded are transmitted with high power from all or a subset of antennas (for example only the 3rd and 4th antennas). As indicated above, these additional pilots 34 are called scheduled pilots. The UE 12 estimates the channel H for data demodulation from these additional pilots 34 in addition to the common pilots 6.
FIG. 6 shows link performance for a system 32 using common pilots for CSI estimation and scheduled pilots for data demodulation. Note that with ideal channel estimation, the performance of the scheduled common pilot solution is always inferior to the common pilot solution with power of −13 dB on the third and fourth antennas. This is due to the additional power allocated to these scheduled pilots. It can also be seen that the performance of scheduled pilots with realistic estimation is close to that of the common pilot solution with pilot power of −13 dB. Hence the scheduled pilot solution is attractive in terms of link performance for a four branch MIMO system.
It can be seen that the performance gains with scheduled pilots are almost negligible at low to medium geometries. Hence the question arises about whether scheduled pilots are needed for all geometries. From FIG. 6 it can be observed that for low to medium geometries/data rates, the common pilot solution is sufficient to give a reasonable performance. Additional pilots are only needed at high signal to noise ratios (SNR) or for high data rate applications.
Conveying Information about Demodulation Pilots
Thus, according to the simulation results shown in FIGS. 2, 3, 4 and 6, common pilots are transmitted for CSI estimation and additional pilots are chosen for data demodulation based on user information available at the Node B. Examples of suitable user information include CSI reports (e.g. CQI, PCI, rank index, RI etc.), user signal quality in general, data rate, service type (e.g. whether the user requires higher data rate or not), geometry (e.g. ratio of received power from the serving cell to that from neighbouring cells) etc. The signal quality can be expressed in terms of for example CQI, SINR, SNR, block error rate (BLER), bit error rate (BER), ACK/NACK for downlink (DL) signal reception, common pilot channel (CPICH) measurements (CPICH received signal code power, CPICH RSCP, CPICH Ec/No) etc.
For example, when the users are nearer to the cell centre (i.e. at high geometries hence when higher order modulations such as 16 quadrature amplitude modulation (QAM) and 64 QAM can be used) additional pilots can be transmitted for data demodulation. Otherwise common pilots are sufficient for data demodulation.
FIG. 7 a signalling diagram indicating how demodulation pilot information is provided to the UE 12. In this figure, common pilots are transmitted from the Node B 4 continuously for CSI estimation (represented as signal 52 in FIG. 7). The UE 12 computes the channel state information channel quality information (CQI), precoding control index (PCI) and rank index (RI) through these channels and reports this information in an uplink feedback channel (e.g. HS-DPCCH), shown as signal 54. Once the Node B 4 receives this information, the scheduler in the Node B 4 decides whether common pilots are needed or scheduled pilots are needed for demodulation.
This can be done based on signal quality (e.g. SNR, SINR etc), user location or the assigned modulation and code rate, etc. If it is decided that the demodulation pilots need to be transmitted, the Node B 4 will convey this information through separate signalling 56 using High Speed-Shared Control Channel (HS-SCCH) orders (i.e. dedicated bit patterns used for switching on demodulation pilots). The HS-SCCH also contains additional information including channelisation code for HS-DSCH, transport format (e.g. MCS) for transmission on HS-DSCH, HARQ feedback for DL transmission etc. If the UE 12 is able to decode this message it will send an ACK to this order to the Node B 4 through the feedback channel (HS-DPCCH), which is shown as signal 58. The common pilots and additional (scheduled) common pilots are then transmitted by the Node B 4 (as shown by signal 62).
The data is transmitted by the Node B 4 to the UE 12 on HS-PDSCH (shown by signal 64). The UE 12 can use the scheduled common pilots for demodulation until again informed by the Node B 4 to use common pilots.
Link Adaptation Using Common Pilots in a MIMO System
FIG. 8 shows a message sequence chart for HSDPA wireless communications. The Node B 4 transmits common pilots with known symbols to the UE 12 through pilot channels 72. The UE 12 computes channel state information (CSI) from the common pilots and conveys this information to the Node B 4 through a feedback channel 74. The Node B 4 processes this information, signals the appropriate control parameters for the data transmission to the UE 12 through a downlink control channel 76 and then sends data to the UE 12 through a downlink data traffic channel 78 according to the determined parameters.
The UE computes the channel state information (CSI) from the common pilots using a link adaptation algorithm. The flow chart in FIG. 9 shows a conventional method of determining CSI information.
In the first step, step 82, the UE computes a signal quality, e.g. the SNR, for each entity in the precoding codebook. The precoding codebook is specified in TS 3GPP 25.212, Version 11.3. For a 4-branch MIMO system, the precoding codebook consists of rank-1, rank-2, rank-3 and rank-4 transmissions, and for a 2-branch MIMO system, the precoding codebook consists of rank 1 and rank 2 transmissions.
Next, in step 84, the UE computes the capacity (C) of each entity in terms of user bit rate, which is determined from a function which maps the downlink signal quality (Q) to the user bit rate (B). A general expression for such a function can be expressed as follows:C=f(αQ)orB=g(αQ)where α is a weighting factor which may depend upon receiver characteristic of the UE. For example more precisely if the signal quality is the SNR, then the capacity can be computed using the formula: capacity, C=log 2 (1+SNR).
In step 86, the UE identifies the precoding control index (PCI) that maximizes the capacity.
For the signal quality (SNR) corresponding to the maximum capacity, the UE performs link adaptation (step 88) to select the modulation and coding scheme (MCS) most suitable for this signal quality (SNR). The selected MCS is then translated into a CSI value (e.g. CQI), which in turn is reported by the UE to the network node. The network node uses the UE reported CSI (e.g. CQI) to select the transport format (e.g. TB size, MCS etc) for scheduling the UE for downlink transmission. For example the transport format related parameters (e.g. modulation type, transport block size, UE identity etc) are conveyed to the UE in a DL control channel (e.g. HS-SCCH in HSPA and PDCCH in LTE) to enable the UE to decode the shared channel which contains the DL transmission (e.g. over HS-DSCH in HSPA or PDSCH in LTE) intended for this UE.
The modulation scheme may be selected from quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (16QAM) or 64 quadrature amplitude modulation (64QAM). In one example the coding scheme can be selected from different coding rates e.g. convolutional coding with rate 1/2, convolutional coding with rate 1/3 etc. In another example the coding scheme can be selected from different types of coding e.g. convolutional coding, turbo coding etc. In yet another example the coding scheme can be selected from a combination of different coding rates and types of coding e.g. convolutional coding 1/2, turbo coding 1/3 etc.
Step 88 is typically performed using lookup tables. An example of such a lookup table is shown in table 1. The network node uses the UE reported CQI to derive the transport format related parameters for scheduling the UE on the downlink for the downlink transmission e.g. over HS-DSCH channel in HSPA or PDSCH in LTE.
TABLE 1Pre-defined table mapping the CQI to transport formatTrans-ReferenceportpowerCQIBlockNumber ofModu-adjustmentvalueSizeHS-PDSCHlationΔNIRXRV0N/AOut of range11361QPSK043200021761QPSK032321QPSK043201QPSK053761QPSK064641QPSK076482QPSK087922QPSK099282QPSK01012643QPSK01114883QPSK01217443QPSK01322884QPSK01425924QPSK01533285QPSK0163576516-QAM0174200516-QAM0184672516-QAM0195296516-QAM0205896516-QAM0216568516-QAM0227184516-QAM0239736716-QAM02411432816-QAM025144241016-QAM026157761064-QAM027217681264-QAM028265041364-QAM029322641464-QAM030322641464-QAM−2Multi-Carrier Deployment
To enhance peak-rates within a technology, multi-carrier or carrier aggregation solutions are known. For example, it is possible to use multiple 5 MHz carriers in High Speed Packet Access (HSPA) to enhance the peak-rate within the HSPA network, and it is intended for Long Term Evolution (LTE) Release 10 to facilitate aggregation of multiple LTE carriers.
In LTE, each carrier in a multi-carrier or carrier aggregation system is generally termed a component carrier (CC) or is sometimes also referred to as a cell. In simple words the component carrier (CC) means an individual carrier in a multi-carrier system. The term carrier aggregation (CA) is also called “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. This means the CA is used for transmission of signalling and data in the uplink and downlink directions. One of the CCs is the ‘primary’ carrier or ‘anchor’ carrier and the remaining ones are called secondary or supplementary carriers. Generally the primary or anchor CC carries the essential UE-specific signalling. The primary CC exists in both uplink and downlink CA. The network may assign different primary carriers to different UEs operating in the same sector or cell.
The CCs belonging to the CA may belong to the same frequency band (aka intra-band CA) or to a different frequency band (inter-band CA) or any combination thereof (e.g. two CCs in band A and 1 CC in band B). The inter-band CA comprising of carriers distributed over two bands is also referred to as dual-band-dual-carrier-HSDPA (DB-DC-HSDPA) in HSPA. Furthermore the CCs in intra-band CA may be adjacent or non-adjacent in the frequency domain (aka intra-band non-adjacent CA). A hybrid CA comprising of intra-band adjacent, intra-band non-adjacent and inter-band is also possible.
In HSPA Release 10, up to four DL carriers can be aggregated as 4C-HSDPA where the DL carriers or DL cells may be belong to the same frequency band or split over two different frequency bands e.g. three adjacent DL carriers in band I (2.1 GHz) and one DL carrier in band VIII (900 MHz). In HSPA Release 11, up to eight DL carriers may be aggregated and may be termed as 8C-HSDPA; and the DL carriers may be distributed over two (or even) more bands. In the present version of the HSPA and LTE specifications (i.e. Release 10) all the carriers that belong to one frequency band have to be adjacent when configured by higher layers (e.g. radio resource control, RRC). In Release 11, non-adjacent carriers within the same band are also possible.
In LTE intra-band CA, in principle up to five DL carriers each of 20 MHz may be aggregated by the UE; at least the UE requirements exist for two DL carriers i.e. up to 40 MHz. In LTE inter-band CA, two DL carriers belonging to two different bands can be aggregated by the UE.